An electronic component on the lower surface of which bumps for connection are provided, such as a flip chip or a CSP (Chip Size Package), are mounted by metallically bonding the bumps to the electrodes of a board. Then, in the mounting process, a gap between the electronic component and the board is generally sealed with a thermosetting resin such as epoxy resin in the mounting process (refer to, for example, Japanese patent application publication No. H06-333985). This has an effect that bonding reliability after the mounting can be secured by reducing thermal stress generated at the bonded portions of the bumps and the electrodes in an electronic-component-mounted board due to a heat cycle during use.
As a method of resin sealing, a method by so-called “postcoating” for injecting a thermosetting resin into the gap between the electronic component and the board after the metal bonding of the bumps and the electrodes (refer to, for example, Japanese patent application publication No. H07-273147) and a method by so-called “precoating” for previously applying a thermosetting resin onto the board prior to the mounting of the electronic component on the board (refer to, for example, Japanese patent application publication No. 2000-58597) have conventionally been known.
However, even when the resin sealing is performed by either the “postcoating” or “precoating” method described above, the following problems occur due to characteristics inherent in each of the methods. First, in the case of resin sealing by “postcoating”, the resin is injected into the gap between the electronic component and the board after the bumps and the electrodes are metallically bonded together. This, therefore, has a defect that a reinforcing effect of the resin sealing cannot be obtained in the metal bonding process.
That is, a thermal stress caused by an expansion coefficient difference between the board and the electronic component takes effect on the bonded portions of the bumps and the electrodes during cooling from the heated state at an elevated temperature in the metal bonding process to the normal temperature, and minute cracks and breakage sometimes occur in the bonded portions before the resin sealing is performed. Such a problem tends to occur as the bumps are reduced in size and pitch in accordance with the downsizing of electronic components and as lead-free solder is increasingly adopted in solder bonding of the bumps. This becomes a big problem in securing the bonding reliability.
Moreover, the resin sealing by “precoating” has a drawback that voids caused by air bubbles entering in the sealing resin tend to occur due to the pressurization of the electronic component against the board in a state in which the resin is applied onto the board. That is, the air bubbles in the gap sometimes remain as voids due to the spread of the resin in a state in which air is confined in the gap between the electronic component and the board, during the process that the resin previously supplied before the component mounting is spread by the lower surface of the electronic component.
Moreover, a solvent constituent and moisture in the resin are gasified during the heating in the metal bonding process, and the gasified gas remains as it is in the resin, similarly generating voids in a case that the resin is confined in the gap between the board and the electronic component in the metal bonding process. Such voids do not only impair the strength of the sealing resin as a resin reinforcement member, but also cause damage of the component as a consequence of breakage due to the expansion of the internal gas when it is heated after mounting. Therefore, it is desired to prevent the occurrence of voids as much as possible.
Furthermore, a resin including filler is often used as the resin material to adjust the physical properties (coefficient of linear expansion etc.) after bonding (curing). However, when such a resin is used for resin sealing by “precoating”, there is problem that so-called “interposing” of the filler interposed between the electrodes and the bumps occurs, impairing the reliability after bonding. Accordingly, there is a restriction that a resin including filler cannot be used in the conventional method of “precoating”.
As described above, it is difficult to sufficiently secure the reinforcing effect by resin sealing even when the resin sealing is achieved by either method of “postcoating” or “precoating” by the electronic component mounting method for metallically bonding the bumps to the electrodes and sealing the mounting gap between the electronic component and the board with resin. Therefore, an electronic component mounting method, which solves the problems and has excellent bonding reliability after mounting, is demanded.
An object of the present invention is to improve the above issues and to provide an electronic component mounting method, which is able to sufficiently secure the reinforcing effect by resin sealing and has excellent bonding reliability after mounting.